1. Field of the Invention
The present invention relates to a method of controlling a roller mill for crushing feedstock of cement, cement clinker or water granulated slug, and a controller for controlling such a roller mill.
2. Description of the Prior Art
The roller mill has the significant advantage over the well-known ball mill that the roller mill is capable of crushing feedstock at a high crushing efficiency (i.e. at a low unit power requirement).
Roller mills are disclosed in Japanese Patent Laid-open (Kokai) Nos. SHO 56-155658 and SHO 57-140658. Hydraulic circuits for use in combination with roller mills are disclosed in Japanese Patent Laid-open (Kokai) Nos. SHO 62-33556, SHO 63-51953, SHO 63-51954, SHO 63-51955, SHO 63-51956 and HEI 2-107346. Separators for roller mills are disclosed in Japanese Utility Model Publication (Kokoku) No. SHO 63-348 and Japanese Patent Laid-open Nos. SHO 59-59855 and SHO 63-42746. A method of regulating the moisture content of feedstock to be processed by the roller mill is disclosed in Japanese Patent Laid-open (Kokai) No. HEI 1-284344. A method of stabilizing the operation of the roller mill is disclosed in Japanese Patent Laid-open No. SHO 59-142856. Roller mills are disclosed in U.S. Pat. Nos. 4,597,537 and 4,611,765.
Since the crushing performance of the roller mill is very sensitive to disturbances such as changes in the crushing characteristics of the feedstock, it is difficult to control the crushing operation of the roller mill in accordance with changes in the nature of the feedstock. Therefore, it is current practice to control the operation of the roller mill on the basis of a skilled operator's qualitative judgment of the condition of the feedstock, as shown in Fig. 7. For example, the skilled operator monitors various operating conditions of the mill, such as product quality and mill vibration, and manipulates the values of directly adjustable operating parameters, such as the rotating speed of the separator, pressure of the working fluid of the hydraulic system, supply power, etc. with the aim of thereby indirectly controlling the operating condition of the mill.
If inappropriate operating parameters are selected and set, problems of the type discussed below can occur.
For example, when vibrations of the roller mill become large and the level of vibrations exceeds a predetermined level, a stopping mechanism is tripped to forcibly stop the operation of the roller mill. The quality of the product produced during the first hour after the roller mill has been restarted is not good and unacceptable products are produced. In the worst case, the roller mill could be damaged by excessive vibrations. Accordingly, the operating parameters for the mill must be determined such that the roller mill will not vibrate excessively and such that the roller mill will not be stopped due to excessive vibrations, in order to prevent mill breakdowns and reductions in the product yield.
In some cases, the roller mill is unable to perform to it's full potential, (which is represented by the quantity of feedstock crushed in unit time, power consumption rate, etc.) and inappropriate operating conditions will result in the deterioration of the quality of the product.
To avoid such troubles, the operation of the roller mill must be continuously monitored by a skilled operator, the training of which requires much time and on-the-job experience. Accordingly, it has been desired to develop a control system capable of automatically and properly controlling the roller mill.
A fuzzy control method that uses fuzzy rules to represent the knowledge of skilled operators is disclosed in Japanese Patent Laid-open (Kokai) No. HEl 2-122848. This fuzzy control method is helpful to solve the above problems. However, even this fuzzy control method has difficulty in properly manipulating many operating parameters such as crushing pressure, crushing rate, rotating speed of the separator, air flow, rotating speed of the table, etc.